LBLRTM Description

(b) spectral variability and noise for the four IASI pixels being averaged

(c) observed - LBLRTM_v11.6 using the radiosonde as input into the calcuation

(d) observed - LBLRTM_v11.6 using the retrieved
atmospheric profile, and

(e) the inital guess (a priori) and retrieved surface emissivity

(plot from [Shephard et al., 2009])

LBLRTM (Line-By-Line Radiative Transfer Model) is an accurate line-by-line
model that is efficient and highly flexible.

LBLRTM attributes provide spectral radiance calculations with accuracies
consistent with the measurements against which they are validated and with
computational times that greatly facilitate the application of the line-by-line
approach to current radiative transfer applications. LBLRTM's
heritage is in FASCODE [Clough et al., 1981, 1992].

Some important LBLRTM attributes are as follows:

the Voigt line shape, computed
with an algorithm based on a linear combination of approximating
functions, is calculated at all atmospheric levels and provides the foundation for
the LBLRTM line shape

modifications to the Voigt are implemented
as needed (e.g. line coupling, continuum) based on analyses of laboratory and
atmospheric spectra

extensively
validated against atmospheric radiance spectra from the ultra-violet to
the sub-millimeter

the self- and
foreign-broadened water vapor continuum model, MT_CKD, as well as
continua for carbon dioxide; among the other continua included in
MT_CKD are the collision induced bands of oxygen at 1600 cm-1 and nitrogen at 2350 cm-1

HITRAN line database
parameters are used including the pressure shift coefficient, the halfwidth temperature
dependence and the coefficient for the self-broadening of water vapor

a Total Internal Partition
Function (TIPS) program is used for the temperature dependence of the line
intensities

CO2 line coupling is
treated as first order with the coefficients for carbon dioxide generated
from the code of Niro et al. (2005) and Lamouroux et al. (2010); CH4 line parameters include line coupling parameters for
the v3 (3000 cm-1) and v4 (1300 cm-1) bands of the main isotopologue

temperature dependent cross section
data such as those available with the HITRAN database may be used to treat
the absorption due to heavy molecules, e.g. the halocarbons

an algorithm is implemented for the
treatment of the variation of the Planck function within a vertically inhomogeneous
layer as discussed in Clough et al. (1992)

algorithmic accuracy of LBLRTM is approximately
0.5% and the errors associated with the computational procedures are of
the order of five times less than those associated with the line parameters
so that the limiting error is that attributable to the line parameters
and the line shape

computational
efficiency mitigates
the computational burden of the line-by-line flux and cooling rate
calculation [Clough et al., 1992], for example linear algebraic
operations are used
extensively in the computationally intensive parts of LBLRTM so that
vectorization
is particularly effective with a typical vectorized acceleration of 20

LBLRTM inputs
are obtained by running the LNFL program
with a line file database for the spectral
lines and cross sections for the heavy molecules. LBLRTM solar inputs
are obtained by running the solar source function
program.